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mm/damon/core: make damon_start() waits until kdamond_fn() starts

PATCH Applied SeongJae Park 1 participant 1 message Dec 8, 2023

Subsystems (3)
Stable branch Memory management DAMON
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SeongJae Park

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PATCH 1/1 mm/damon/core: make damon_start() waits until kdamond_fn() starts

The cleanup tasks of kdamond threads including reset of corresponding DAMON context's ->kdamond field and decrease of global nr_running_ctxs counter is supposed to be executed by kdamond_fn(). However, commit 0f91d13366a4 ("mm/damon: simplify stop mechanism") made neither damon_start() nor damon_stop() ensure the corresponding kdamond has started the execution of kdamond_fn().

As a result, the cleanup can be skipped if damon_stop() is called fast enough after the previous damon_start(). Especially the skipped reset of ->kdamond could cause a use-after-free.

Fix it by waiting for start of kdamond_fn() execution from damon_start().

Fixes 0f91d13366a4 ("mm/damon: simplify stop mechanism")
Reported-by Jakub Acs <[email protected]>
Cc <[email protected]> # 5.15.x
Signed-off-by SeongJae Park <[email protected]>

---
Note that the report has not publicly made, so this patch doesn't have a
Closes: tag.

 include/linux/damon.h | 2 ++
 mm/damon/core.c       | 6 ++++++
 2 files changed, 8 insertions(+)
include/linux/damon.h +2 -0
index aa34ab433bc5..12510d8c51c6 100644
@@ -579,6 +579,8 @@ struct damon_ctx {
 	 * update
 	 */
 	unsigned long next_ops_update_sis;
+	/* for waiting until the execution of the kdamond_fn is started */
+	struct completion kdamond_started;
 
 /* public: */
 	struct task_struct *kdamond;
mm/damon/core.c +6 -0
index f91715a58dc7..2c0cc65d041e 100644
@@ -445,6 +445,8 @@ struct damon_ctx *damon_new_ctx(void)
 	if (!ctx)
 		return NULL;
 
+	init_completion(&ctx->kdamond_started);
+
 	ctx->attrs.sample_interval = 5 * 1000;
 	ctx->attrs.aggr_interval = 100 * 1000;
 	ctx->attrs.ops_update_interval = 60 * 1000 * 1000;

@@ -668,11 +670,14 @@ static int __damon_start(struct damon_ctx *ctx)
 	mutex_lock(&ctx->kdamond_lock);
 	if (!ctx->kdamond) {
 		err = 0;
+		reinit_completion(&ctx->kdamond_started);
 		ctx->kdamond = kthread_run(kdamond_fn, ctx, "kdamond.%d",
 				nr_running_ctxs);
 		if (IS_ERR(ctx->kdamond)) {
 			err = PTR_ERR(ctx->kdamond);
 			ctx->kdamond = NULL;
+		} else {
+			wait_for_completion(&ctx->kdamond_started);
 		}
 	}
 	mutex_unlock(&ctx->kdamond_lock);

@@ -1483,6 +1488,7 @@ static int kdamond_fn(void *data)
 
 	pr_debug("kdamond (%d) starts\n", current->pid);
 
+	complete(&ctx->kdamond_started);
 	kdamond_init_intervals_sis(ctx);
 
 	if (ctx->ops.init)
Signature · 1 line
2.34.1